Toxoplasma gondii strain T-263 is an oocyst negative genetic mutant that induces immunity in cats to wild type strains when tissue cysts or bradyzoites are administered orally. The vaccine potential of T-263 has been proven in two separate efficacy studies. Use of this feline vaccine will reduce oocyst contamination of homes and surroundings thereby reducing zoonotic toxoplasmosis in humans. Production of the vaccine in mice is economically and practically unacceptable. Thus, commercialization depends on the development of an economic method for the production of tissue cysts/bradyzoites. The overall objective of this proposal is to demonstrate the feasibility of producing T-263 tissue cysts (containing bradyzoites) in cell culture. A laboratory-scale cell culture system will be optimized for maximal cyst production by using strategies to prevent cell destruction by tachyzoites and to enhance cyst persistence. The feasibility of genetic selection for enhanced tissue cyst formation in cell culture will also be evaluated. This will lead directly into the Phase II program of process scale-up and down-stream processing development. This novel production technology will have immediate application to the commercialization of the T-263 vaccine.